Application of metal hydrides (MH) is a very promising way to solve the problem of hydrogen recovery from process off-gases. The selectivity of reversible hydrogen interaction with hydride-forming materials allows the development of systems for hydrogen extraction from gas mixtures and its purification which are simple in layout and operation.
The main problem that hampers the realisation of this approach is in the deterioration of MH performances caused by gas impurities, in particular, oxygen and water vapours. This is mainly caused by slow hydrogen dissociation on the oxidized surface that was shown to be a very important partial step playing a substantial role in activation and passivation of hydrogen absorption reactions at ambient temperatures. That is why enhancement of the hydrogen sorption performances requires catalysis of the hydrogen exchange surface processes, including dissociation of H2 molecules in the course of hydrogenation (hydrogen absorption) and recombination of H atoms during dehydrogenation (hydrogen desorption).
Surface modification of the metallic hydride-forming materials by Platinum Group Metals (PGM), enhances the overall poisoning resistance of the alloys towards many aggressive surface adsorbates (e.g. H2S, CO, CO2, O2, H2O) and provides higher H2 absorption/desorption rates. PGM, including Palladium and Platinum, are excellent catalysts for hydrogen exchange reactions. PGM catalysts are known to lower the activation energy needed for breaking the H—H bond in H2 molecules. The H atoms are then free to react at the PGM catalyst or hydrogen absorbent surface or leave the surface to participate in reactions elsewhere in the chemical system. PGM surface coatings enable hydrogen to pass rapidly through the surface of the MH to the bulk, while still maintaining the hydrogenation activity. PGM coatings have been observed to produce large increases in the activation kinetics of MH materials at room temperature as well as extending their cycle lifetimes.
Surface modification of the hydride forming metals and alloys by the deposition of PGM onto the surface of their particles is a known approach documented in a number of patents. The most typical ones are briefly described below:                According to U.S. Pat. No. 4,468,235 (Hill), a hydride forming titanium alloy was coated by a hydrogen-permeable metal (including Pd) that allowed usage of the coated alloy for hydrogen separation and purification; the metal coating was made by anodic etching of the alloy followed by electroless plating.        U.S. Pat. No. 6,165,643 (Doyle et al) discloses a hydrogen storage material comprising hydride-forming metallic particles whose surface has a discontinuous or partial deposit of one or more platinum group metals (e.g., Pd, Ru); the PGM deposition (0.08 to 2 wt. %) is carried out from aqueous solutions of their salts which can or cannot contain a reducing (hydrazine hydrate, sodium hypophosphite, etc.) and complexing (EDTA) agents; in the latter case the PGM reduction from the salts takes place by hydrogen desorbed from the preliminary hydrogenated substrate material.        Similarly, U.S. Pat. No. 5,766,688 (Law, Vyas) discloses a procedure for activation of metal hydrides that includes cathodic charging of a hydride forming material with hydrogen in aqueous electrolyte (this process is accompanied by the material fracturing to form a powder) followed by its plating with a metal (including Pd) to be reduced from the metal salt by the hydrogen from the metal hydride substrate.        
The quality of the deposited PGM layers is paramount to the absorption performance and poisoning resistance of the surface-modified hydride forming metals and alloys. In turn, it is strongly dependent on the adhesion between the PGM particles and the alloy surface. In most combinations “PGM-alloy substrate” the adhesion is rather poor, resulting in a poor coating quality, loss of appreciable quantities of PGM metals, and deterioration of hydrogen absorption/desorption performances in the surface-modified materials.
A common approach which could be adopted in the surface modification of hydride forming alloys for use as a hydrogen separation/purification medium is an electroless deposition of PGM-based metal layers. This is a wet chemical reduction process in which aqueous metal ions, in alkaline or acidic baths, are auto-catalytically reduced at a solid-liquid interface in the absence of an applied external electrical current to activate the process. Most PGMs can be plated onto surfaces using this technique. Its advantages include low cost and simplicity, flexibility as to the state of the substrate (both monolith and powdered materials can be modified), high quality of the covering layers (uniform thickness, low porosity), the ability to plate non-conductive surfaces, etc. Catalysis of the electroless plating process is typically achieved through activation of the substrate in, e.g., a PdCl2 solution. Pd2+ ions are well known to be reduced on the colloidal Sn2+ ions sensitized onto the surface of the substrate, although this physical adhesion is weak in nature and the resulting Pd nuclei are generally detached and lost in solution (i.e. bath decomposition). During sensitization the Sn2+ ions attach themselves to the substrate surface oxide or hydroxyl groups. Activation then continues by Pd2+ reduction into Pd0 nuclei assisted by Sn2+ oxidation. The Pd0 species take on the role of reaction catalysts after the palladium deposition was allowed to commence on the activated substrate surface.
An obvious way for the realization of this approach may be in the use of multiple electroless deposition steps (substrate surface cleaning-sensitization/activation-autocatalytic reduction in a plating bath).
The conventional electroless plating is a costly and time-consuming exercise, and the preparation of uniform surface coatings is not guaranteed. Furthermore, the oxide layer on the surface of the core material inhibits interactions with PGM precursor ions in solution. Finally, PGM (e.g., Pd, Pt) colloidal particles have a poor adhesion to the substrate because of the absence of chemical conjunction. As a result, the PGM activation particles leach out into the solution, and further autocatalytic reduction of the noble metals takes place in the plating bath, rather than on the substrate surface. The end result is in the decomposition of the plating bath and the loss of the expensive plating agent.
It is an object of the invention to suggest a method for surface modification of metallic hydride-forming materials tag which will assist in overcoming the aforementioned problems.